Microchip laser

ABSTRACT

A gain medium is disposed between two mirrors to form a resonant cavity. The cavity length is selected so that the gain bandwidth of the gain medium is less than or substantially equal to the frequency separation of the cavity modes and such that a cavity mode frequency falls within the gain bandwidth. A nonlinear optical material is disposed either inside or outside the cavity to generate new laser wavelengths. The nonlinear optical material may be contained in a cavity which is resonant at the microchip laser frequency. Alternatively, the microchip laser may be tuned, for example thermally or by the application of a longitudinal or transverse stress, to the frequency of the resonant cavity. The laser is optically pumped by any appropriate source such as a semiconductor injection laser or laser array. Suitable gain media include Nd:YAG, Nd:GSGG and Nd pentaphosphate, and suitable non-linear optical material include MgO:LiNbO 3  and KTP.

BACKGROUND OF THE INVENTION

This application is a continuation-in part of U.S. Ser. No. 151,396filed Feb. 2, 1988, now U.S. Pat. No. 4,860,304 issued Aug. 22, 1989.

This invention relates to single frequency microchip lasers.

In this specification, numbers in brackets refer to the referenceslisted at the end of the specification, the teachings of which areincorporated herein by reference. The realization of practicalsingle-frequency, diode-pumped, solid-state lasers has been the goal ofseveral researchers over the past 20 years. For a complete review ofdiode pumped solid-state lasers see T. Y. Fan and R. L. Byer, IEEE J.Quantum Electron 6, 895 (1988). One approach has been the solid-state,unidirectional, nonplanar, ring oscillator. See, T. J. Kane, A. C.Nilsson, and R. L. Byer, Opt. Lett. 12, 175 (1987). While this approachprovides the desired laser characteristics, it suffers from acomplicated fabrication process and optical alignment is critical. Asimpler approach is the miniature, linear, solid-state cavity. See, B.Zhou, T. J. Kane, G. J. Dixen, and R. L. Byer, Opt. Lett. 10, 62 (1985),A. Owyoung, G. R. Hadley, R. Esherick, R. L. Schmidt, and L. A. Rahn,Opt. Lett. 10, 484 (1985) and K. Kubodera and J. Noda, Appl. Opt. 12,3466 (1982). Although there has been some work on multimode miniatureflat-flat cavities, G. Winter, P. G. Mockel, R. Oberbacher, and L. Vite,Appl. Phys. 11, 121 (1976), the most common design for single-modeminiature cavities uses one curved mirror to stabilize the resonator.See, the B. Zhou, A. Owyoung and K. Kubodera references set forth above.In allowed U.S. patent application Ser. No. 151,396, filed Feb. 2, 1988,now U.S. Pat. No. 4,860,304 issued Aug. 22, 1989, there is disclosed asolid-state, optically pumped microchip laser in which the cavity lengthis selected so that the gain bandwidth of the gain medium is less thanthe frequency separation of the cavity modes. This relationshipguarantees that only a single longitudinal mode will oscillate when thefrequency of this mode falls within the laser gain region.

SUMMARY OF THE INVENTION

The solid-state, optically pumped microchip laser according to oneaspect of the invention includes a solid-state gain medium disposedbetween two mirrors, the distance between the mirrors selected so thatthe gain bandwidth of the gain medium is substantially equal to thefrequency separation of the cavity modes. In another aspect, asolid-state gain medium is disposed between two mirrors, the distancebetween the mirrors selected so that the gain bandwidth of the gainmedium is less than or substantially equal to the frequency separationof the cavity modes. A nonlinear optical material is disposed to receivelight from the gain medium, the nonlinear optical material selected togenerate second or higher harmonics of the light from the gain medium.

In yet another aspect of the invention, the microchip laser includes asolid-state gain medium/nonlinear optical material combination disposedbetween two mirrors, the distance between the mirrors selected so thatthe gain bandwidth of the gain medium is less than or substantiallyequal to the frequency separation of the cavity modes. The nonlinearoptical material is selected to generate second or higher harmonics ofthe light from the gain medium.

By selecting the cavity length so that the gain bandwidth issubstantially equal to the frequency separation of the cavity modes, oneis guaranteed that only one cavity frequency falls within the laser gainregion and only one laser frequency will oscillate. The inclusion ofnonlinear optical material provides light in the visible or ultravioletregions useful for read and write optical disks and for projectiontelevision applications, among others. Both the laser gain element andthe nonlinear crystal are dielectrically coated flat wafers. Thesewafers are bonded together with transparent optical cement and dicedinto many small sections which greatly reduces the cost and complexityof such lasers as compared with devices using discreet opticalcomponents that are fabricated and assembled separately.

The single frequency microchip lasers according to the invention employa miniature, monolithic, flat-flat, solid-state cavity whose modespacing is greater that the medium gain bandwidth. These lasers rely ongain-guiding or nonlinear optical effects to define the transversedimensions of the lasing mode. As a result of the monolithic, flat-flatconstruction, the fabrication process for the microchip laser lendsitself to mass production. The cost per laser is extremely low becauseof the small amount of material used for each laser and the simplefabrication. The resulting microchip lasers are longitudinally pumpedwith the close-coupled, unfocused output of a diode laser.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1a and 1b, are graphs of laser gain and oscillation modes versusfrequency;

FIGS. 2a and 2b are cross-sectional views of a microchip laser of theinvention;

FIG. 3 is a graph of output intensity versus wavelength;

FIG. 4 is a graph of output power versus pump power for lasers of theinvention;

FIGS. 5a and 5b are graphs illustrating measured spectral response ofthe lasers of the invention;

FIG. 6 is a cross-sectional view of an embodiment of the inventionincluding a nonlinear optical element;

FIG. 7 is a cross-sectional view of an embodiment of the invention witha nonlinear optical element incorporated within the laser resonantcavity;

FIG. 8a depicts an array of microchip lasers on a wafer in associationwith a wafer of diode pump lasers;

FIG. 8b depicts the array of FIG. 8a in association with a wafer ofnonlinear optical material with Fabry-Perot resonators; and

FIG. 9a depicts an embodiment of the microlaser of FIG. 6 with anapparatus for stress tuning and

FIG. 9b depicts an embodiment of the microlaser of FIG. 6 with anapparatus for thermal tuning.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The theory on which the present invention is based will now be discussedin conjunction with FIG. 1. In FIG. 1a, a curve 10 is a plot of gainversus frequency for any solid-state laser gain medium such as Nd:YAG orNd pentaphosphate. The gain bandwidth ν_(g) of the curve 10 is definedas the separation between arrows 12 and 14 wherein the gain exceeds theloss. Also shown in FIG. 1a are intracavity modes 16 and 18. Theseparation ν_(c) between adjacent cavity modes is given by the equationν_(c) =c/2nl where c is the speed of light, n is the refractive index ofa gain medium and l is the length of a resonant cavity. As shown in FIG.1a, a cavity length l has been selected so that l is less than c/2nν_(g)resulting in the intracavity modes 16 and 18 being spaced greater thanthe gain bandwidth of the curve 10 and the absolute frequency of thecavity mode ν_(a) =mc/2nl where m is an integer such that the frequencyfalls outside the gain bandwidth. In the case illustrated theintracavity modes 16 and 18 straddle the gain curve 10 so that therewill be no lasing of the gain medium since there is no overlap of thegain curve 10 with either of the modes 16 or 18. To insure that the gainmedium will laser, it is necessary that there be at least some overlapof the gain curve 10 with one of the intracavity modes such as the mode18 as shown in FIG. 1b. Assuring such overlap is accomplished by anappropriate choice of gain material and cavity length.

With reference now to FIG. 2a, a microchip laser 30 includes asolid-state gain medium 32 disposed between a pair of mirrors 34 and 36.The mirrors 34 and 36 are coated with multiple layers (20-30 layers) ofdielectric material. The gain medium 32 is pumped optically by a laser38 whose output light 40 is focused by a lens 42 onto the mirror 34. Themirror 34 transmits light from the pump laser 38 but reflects lightgenerated within the gain medium 32. The length l of the gain medium 32is selected so that l≦c/2nν_(g) where ν_(g) is the bandwidth of the gainmedium. In this case, as pointed out above, a single mode only willoscillate within the gain medium 32 when ν_(a) falls within the gainbandwidth so that the output light 44 from the laser 30 is singlefrequency. The mirrors 34 and 36 may be separate elements bondeddirectly to the gain medium 32 or they may be multilayer coatingsdeposited directly on the opposing flat surfaces of the gain medium 32.In FIG. 2b, the laser 38 is placed close to or bonded directly to themirror 34 so that most of the light from the pump laser is absorbed inthe fundamental mode region of the microchip laser.

To demonstrate the feasibility of diode-pumped microchip lasers, severaldifferent microchip lasers were constructed and operated CW at roomtemperature. These included: Nd:YAG (Nd_(x) Y_(3-x) Al₅ O₁₂) at 1.06 μmusing a 730-μm-long cavity; Nd:YAG at 1.3 μm using a 730-μm-long cavity;Nd pentaphosphate (NdP₅ O₁₄) at 1.06-μm using a 100-μm-long cavity; andNd:GSGG (Nd_(x) Gd_(3-x) Sc₂ Ga₃ O₁₂) at 1.06 μm using a 625-μm longcavity. In each case, single-longitudinal-mode, single-spatial-modeoperation was achieved with pump powers many times above threshold.

The performance of the 1.06 μm Nd:YAG microchip lasers will now bediscussed. These lasers were constructed from a slab of YAG with 1.1 wt.percent Nd doping. The slab was cut and polished to a thickness of 730μm. Dielectric cavity mirrors were deposited directly onto the YAG. Onother microchip lasers the mirrors were cut from 100-μm-thick wafermirrors and then bonded to the Nd:YAG. The performance of theseparate-mirror devices was very similar to the performance of thedielectrically coated Nd:YAG cavities. The output mirror 36 had areflectivity of 99.7% at 1.06 μm and was designed to reflect the pumplaser. The opposite mirror 34 had a reflectivity of 99.9% at 1.06 μm andtransmitted the pump. The Nd:YAG was cut into pieces 1 mm square (orless) and bonded to a sapphire heat sink (not shown). Damage to thedielectric coatings from cutting the wafers was confined to a distanceof less than 30 μm from the edge of the chips.

A Ti:Al₂ O₃ laser was used as a pump source to characterize themicrochip lasers prior to diode pumping. It was tuned to the Nd:YAGabsorption peak at 0.809 μm and focused onto the microchip laser, withan experimentally determined spot size of about 50 μm in the Nd:YAGcrystal. Measurements showed that 18% of the incident pump power wasreflected by the laser package and 27% was transmitted. The efficiencyof the microchip lasers can be improved with better dielectric coatings.

When the Nd:YAG microchip laser was properly aligned with the pump,single-longitudinal-mode, single-spatial-mode operation was observed.The output beam 22 was circularly symmetric with a divergence of about20 mrad, determined by the spot size of the pump. Spectrometer traces(FIG. 3) showed only single-longitudinal-mode operation for absorbedpump powers up to 40 mW. The lasing frequency tuned slightly as the pumpspot on the microchip cavity was moved to positions with a slightlydifferent cavity length. The devices constructed with wafer mirrors werecontinuously tunable over the entire gain spectrum by mechanicalmovement of the mirrors In contrast to results reported in P. Esherickand A. Owyoung, Technical Digest Conference on Lasers and Electrooptics(Optical Society of America, Washington, DC, 1988), paper THB2. Theoutput polarization of the microchip laser in the same direction as thepolarization of the pump to better than 1 part in 100.

A computer-controlled variable attenuator was introduced into the pathof the pump beam to obtain the input-output power characteristics of themicrochip laser. The lasing threshold was measured to be below 1 mW, andthe slope quantum efficiency (determined from the output of the laserfrom the 99.7% reflecting mirror only) was slightly greater than 30%.The input-output curve is shown in FIG. 4. At higher pump powers thermaleffects led to unrepeatable results. The highest single-mode CW outputpower achieved with the microchip laser was 22 mW.

The linewidth of the Nd:YAG microchip lasers was measured byheterodyning two free-running devices together. Thermal tuning was usedin order to get the lasers to operate at nearly the same frequency. Theoutputs of the lasers were stable enough to obtain heterodynemeasurements with a resolution of 10 kHz. At this resolution, themeasured spectral response was instrument limited. (See FIG. 5) Thisgives a linewidth for the microchip lasers of less than 5 kHz, assumingequal contributions to the linewidth from each laser. The theoreticalphase fluctuation linewidth is estimated to be only a few hertz.Relaxation oscillations account for the observed sidebands 700 kHz awayfrom the main peak. The intensity of the sidebands varied with time, butwas always greater than 30 dB below the main peak.

The microchip Nd:YAG lasers have been pumped with the unfocused outputof a 20-mW GaAlAs diode laser. The Nd:YAG cavity was placed about 20 μmfrom the output facet of the diode laser and longitudinally pumped. Theresulting pump spot size in the Nd:YAG was about 50 μm in diameter. Theoutput of the microchip laser showed single-longitudinal-mode,fundamental (i.e. lowest order) single-spatial-mode operation at allavailable powers. The divergence of the laser was diffraction limited atabout 20 mrad.

Important embodiments of the present invention are shown are FIG. 6. InFIG. 6a a dielectrically

coated flat wafer 50 of a nonlinear optical material is located toreceive light from the microchip laser 30. The wafer 50 includesdielectric coatings 52 and 54. The nonlinear optical material of thewafer 50 has the property that, when exposed to monochromatic light, itgenerates a beam of light including harmonics of the incident beam.Suitable nonlinear optical materials are, for example, MgO:LiNbO₃ andKTP (potassium titanyl phosphate). As in the embodiments of FIG. 2, thecavity length l of the gain medium 32 satisfies the relationshipl≦c/2nν_(g). Light from the microchip laser 30 passes into the nonlinearoptical element 50 which shifts the frequency to one of the harmonics ofthe incident beam. A particularly useful harmonic is the secondharmonic. The optical coatings 52 and 54 are chosen such that they forma Fabry-Perot cavity at the pump wavelength. Typical reflectivities ofsuch coatings at the pump wavelength are 98%. Mirror 52 is also highlyreflective at the harmonic wavelength while mirror 54 is highlytransmissive at the harmonic wavelength. In addition to techniqueswhereby the cavity frequency of the harmonic crystal is tuned to thelaser frequency, the single-frequency microchip laser may be tuned to beresonant with any of the harmonic crystal cavity modes and may be lockedto that frequency in any number of ways including monitoring of theintensity of the harmonic output power. The microchip laser may becontinuously tuned by a number of techniques that are well knownincluding the application of a longitudinal or transverse stress to thecrystal or by modifying the refractive index of the crystal thermally.

FIG. 9a shows a microlaser 30 associated with a nonlinear opticalmaterial 50 located within a Fabry-Perot cavity made up of mirrors 52and 54. The gain medium 32 of the laser 30 is positioned between a fixedstop 912 and a movable stop 910. Pressure is applied (shown here asbeing applied by an adjustable screw 914) to the gain medium 32 throughthe movable stop 910. By adjusting the pressure on the gain medium 32,the frequency of the laser light can be matched to the resonantfrequency of the Fabry-Perot cavity. In FIG. 9a a transverse stress isbeing applied to the medium. It is possible to stress tune the laser bythe application of a longitudinal stress along the direction of thelaser light.

FIG. 9b shows a microlaser 30 associated with a nonlinear opticalmaterial 50 as in FIG. 9a. However, in this embodiment the gain material32 is positioned within a temperature regulating jacket 918 which can beheated or cooled by the temperature regulating elements 916. Byadjusting the temperature of the gain medium 32 the frequency of thelaser light can be thermally tuned to match the resonant frequency ofthe Fabry-Perot cavity. Because the laser frequency can be changed bythermal tuning, it should also be noted that in order to stress tune amicrolaser and have it remain tuned, it may be necessary to regulate thetemperature of the gain medium.

The ability to continuously tune the microchip laser over its gainbandwidth without a mode jump is a significant advantage in being ableto precisely tune and lock to any of the Fabry-Perot cavity modes of theharmonic crystal.

The harmonic crystal with its resonant cavity may be separate from themicrochip laser or it may be bonded directly to the output end of themicrochip laser using an optically transparent cement. The use offlat-flat cavities on the harmonic crystal simplify the fabricationprocess by using similar wafer processing technology as that for themicrochip laser. However, any of the well known techniques for aresonant harmonic cavity may also be used in conjunction with themicrochip laser such as the unidirectional ring resonator or sphericalmirror cavity. Further, the nonlinear material may be incorporatedwithin the laser cavity itself.

FIG. 6(b) shows a configuration similar to FIG. 6(a) but with the diodeplaced close to or bonded to the laser medium. FIG. 7 is an embodimentof the invention in which nonlinear optical material forms a part of thelaser cavity structure. A microchip laser 70 includes a flat wafer 72 ofan active gain medium. A nonlinear optical element 74 is bonded to thegain medium 72. Dielectric mirrors 34 and 36 complete the microchiplaser 70. The length l between the mirrors 34 and 36 satisfies therelationships

    ν.sub.g ≦c/2(n.sub.1 1.sub.1 +n.sub.2 1.sub.2)

    l=l.sub.1 +l.sub.2

Where l₁, n₁ are the length and index of refraction respectively of thegain medium and l₂, n₂ are the length and index of refraction of thenon-linear material.

It will be appreciated by those skilled in the art that by selection ofthe appropriate nonlinear optical material, the output from theharmonically converted microchip laser can be in the visible orultraviolet region and be useful for read and write optical disks orprojection television applications. It will also be appreciated thatusing the same fabrication techniques, an electro-optic or acousto-opticmodulator can be incorporated into the composite structure with themodulator electrodes being photolithographically incorporated onto thewafers before being diced up. Such fabrication techniques would greatlyreduce the cost and complexity of such devices over those using discreteoptical components that are fabricated and assembled separately.

In addition to harmonic generation by means of a suitable nonlinearmaterial, nonlinear frequency conversion may be carried out in suitablenonlinear optical materials using optical parametric oscillation oramplification as well as frequency sum or difference mixing using thesingle frequency microchip laser. Similar cavity fabrication as thatdescribed above may be used to create a microchip laser whose singlefrequency light is frequency converted by parametric conversion intolight of two lower frequencies. In this parametric conversion microchiplaser, the resonators differ from those previously described only inthat the nonlinear optical material is a parametric conversion materialsuch as LiNbO₃ or KNbO₃ and the cavity coatings are chosen according tothe well known art form for such devices.

Fabrication of the foregoing embodiments will now be discussed. A bouleof laser material is grown by conventional methods. The boule is slicedto the appropriate thickness depending on whether or not a nonlinearoptical element is to be incorporated into the resonant cavity. Theresulting wafer is lapped to the desired length, parallel and flat tobetter than one wavelength using conventional lapping techniques. Thewafer must be flat over the area being irradiated by the diode pumplaser.

At this point, the gain medium is coated with multiple layers ofdielectric material on opposite faces. Alternatively, separate mirrorsare bonded to the gain medium. The slice or wafer is then diced intoindividual microchip lasers which can be mounted to the pump optics.

The nonlinear optical elements used for frequency shifting are lappedflat and parallel and incorporated either within or outside of the lasercavity as shown in FIGS. 6 and 7. In the case in which the nonlinearoptical material is outside the resonant cavity, both surfaces of thenonlinear material are coated with dielectric layers which are highlyreflective to the frequency of light generated by the microchip laser.In addition, the surface of the nonlinear optical material nearest tothe microchip laser is coated with dielectric layers which are highlyreflective of the wavelength of light generated by the nonlinear opticalmaterial. Although for ease of manufacture, the nonlinear element may belapped with both surfaces flat, it is also possible, in the case of theoptical material being outside the laser cavity, to have the surfaceaway from the microchip laser be a nonflat surface.

Any non-parallel resonator structure known to the art of resonators maybe used with the microchip laser. In fact, the use of the microchiplaser makes such resonator design easier, since the laser can be tunedto the cavity frequency.

If a nonlinear optical element is to be part of the resonant cavity, theboule of gain medium is sliced and lapped flat and parallel. Thenonlinear optical material is also lapped flat and parallel and bondedto the gain medium using transparent optical cement. The lengths of thegain material and the nonlinear optical material satisfy therelationship discussed above. After the gain medium and nonlinearoptical material are bound, mirrors are applied to the other surfaceseither as multilayer dielectric coatings on the material itself or asseparate mirrors, and the wafers are diced into chips.

It should be noted that one can construct a microchip laser array or amicrochip laser nonlinear frequency converter array by simply not dicingthe wafers. That is, by leaving the microchip laser in wafer form, andassociating the microchip laser wafer with a two dimensional diode laserarray to pump the microchip laser array, a two dimensional array ofmicrochip lasers is immediately formed. Referring to FIG. 8a, a twodimensional array of microchip lasers consists of an array of laserdiodes 810 associated with a microchip laser wafer 812. Light from thelaser didoes 814 excites the microchip laser to emit light. The twowafers can be cemented together using optically transparent cement. Ifthe microchip laser wafer also contains the nonlinear optical materialwithin its resonant cavity, frequency conversion using a two wafersystem is possible. If, however, the nonlinear optical material is noton the microchip laser, an array can still be constructed by simplyplacing the wafer with the nonlinear optical material and Fabry-Perotresonant cavity 816 in association with the other wafers. Such aconfiguration is shown in FIG. 8b. The resonant frequency of thenonlinear optical material cavity must be matched to the microchipfrequency in the manner described above.

A microchip laser array is particularly useful if a directed laser beamis required. By properly modulating the phase of each microchip laserusing, for example, individually addressable phase modulators placed inthe output beam or within the cavity of each microchip laser, it ispossible to phase steer the array and thereby direct the beam. Theoutput of the two-dimensional array of single-frequency microchip lasersmay be coherently combined into a single beam using well knowntechniques of binary optics.

The diode-pumped microchip lasers according to the invention exhibit lowpump threshold, high efficiency, and single frequency operation. Theselasers can be continuously tuned across their gain-bandwidth in a singlefrequency using a transversely or longitudinally applied stress to themicrochip laser crystal. The microchip laser is applicable, for example,to frequency converters, modulators and Q-switches, withphotolithographically deposited electrodes. It results in low cost,volume-producible lasers and electro-optic devices. The incorporation ofnonlinear optical material, either inside or outside of the cavity,generates other wavelengths of light. The output from such a device, inthe visible or ultraviolet region, is useful for read and write opticaldisks and projection television applications.

What is claimed is:
 1. Solid state, optically pumped microchip lasercomprising:solid state gain medium disposed between two mirrors, thedistance between the mirrors selected so that the gain bandwidth of thegain medium is less than or substantially equal to the frequencyseparation of the cavity modes and such that one cavity mode frequencyfalls within the gain bandwidth of the medium; and nonlinear opticalmaterial disposed to receive light from the gain medium, the nonlinearoptical material selected to generate second or higher harmonics of thelight from the gain medium, said nonlinear optical material containedwithin a Fabry-Perot resonator.
 2. The microchip laser of claim 1wherein said microchip laser further comprises an apparatus for applyinga longitudinal stress to said gain medium to thereby tune a frequency ofthe light from the microchip laser to be coincident with a resonantfrequency of the Fabry-Perot resonator containing a suitable nonlinearcrystal.
 3. The microchip laser of claim 1 wherein said microchip laserfurther comprises an apparatus for applying a transverse stress to saidgain medium to thereby tune a frequency of the light from the microchiplaser to be coincident with a resonant frequency of the Fabry-Perotresonator containing a suitable nonlinear crystal.
 4. The microchiplaser of claim 1 wherein said microchip laser further comprises anapparatus for changing the temperature of said gain medium to therebytune a frequency of the light from the microchip laser to be coincidentwith a resonant frequency of the Fabry-Perot resonator containing asuitable nonlinear crystal.
 5. The microchip laser of claim 1 whereinthe nonlinear optical material is contained within a resonator withplanar parallel faces.
 6. The microchip laser of claim 1 wherein thenonlinear optical material is contained within a resonator having a flatface disposed toward the gain medium and a spherical face disposed awayfrom the gain medium.
 7. Solid state, optically pumped microchip lasercomprising:a solid state gain medium and nonlinear optical materialcombination disposed between two mirrors, the distance between themirrors selected so that the gain bandwidth of the gain medium is lessthan or substantially equal to the frequency separation of the cavitymodes and such that one cavity mode frequency falls within the gainbandwidth of the medium, the nonlinear optical material selected togenerate other frequencies from the light from the gain medium.
 8. Themicrochip laser of claim 7 wherein the length of the gain medium and thelength of the nonlinear material satisfies the relationship

    ν.sub.g ≦C/2(n.sub.1 l.sub.1 +n.sub.2 l.sub.2)

wherein n₁ and l₁ are the refractive index and length, respectively, ofthe gain medium, n₂ and l₂ are the refractive index and length,respectively, of the nonlinear optical material, and ν_(g) is thebandwidth of the gain material.
 9. The microchip laser of claim 1, 2, or7 in which the gain medium is Nd:YAG.
 10. The microchip laser of claim 9in which the distance between the mirrors is about 730 μm.
 11. Themicrochip laser of claim 1, 2, or 7 in which the gain medium is Ndpentaphosphate.
 12. The microchip laser of claim 11 in which thedistance between the mirrors is about 100 μm.
 13. The microchip laser ofclaim 1, 2, or 7 in which the gain medium is Nd:GSGG.
 14. The microchiplaser of claim 13 in which the distance between the mirrors is about 625μm.
 15. The microchip laser of claim 9 in which the optical pumping istuned to 0.809 μm.
 16. The microchip laser of claim 1, 2, or 7 in whichthe mirrors are formed of multiple layers of dielectric.
 17. Themicrochip laser of claims 1, 2, 7, 9, 11 or 13 in which the nonlinearmaterial is MgO:LiNbO₃.
 18. The microchip laser of claims 1, 2, 7, 9, or13 in which the nonlinear material is KTP.
 19. The microchip laser ofclaim 15 in which the optical pumping source focuses onto the Nd:YAGcrystal to a spot size of 50 μm.
 20. Solid state, optically pumpedmicrochip laser comprising:a solid state gain medium disposed betweentwo mirrors, the distance between the mirrors selected so that the gainbandwidth of the gain medium is substantially equal to the frequencyseparation of the cavity modes and such that one cavity mode frequencyfalls within the gain bandwidth of the medium.
 21. The laser of claim 20further comprising an apparatus adapted to change the temperature ofsaid gain medium to thereby thermally tune said laser.
 22. The laser ofclaim 20 further comprising an apparatus adapted to apply a longitudinalstress to said gain medium to thereby stress tune said laser.
 23. Thelaser of claim 20 further comprising an apparatus adapted to apply atransverse stress to said gain medium to thereby stress tune said laser.24. An array of microchip lasers comprised of a wafer of gain materialdisposed between two mirrors, the thickness of the wafer selected sothat the gain bandwidth of the gain medium is less than or substantiallyequal to the frequency separation of the cavity modes and such that onecavity mode frequency falls within the gain bandwidth of the medium; andpositioned adjacent to a wafer of diode lasers aligned so as tostimulate said gain medium into light emission.
 25. The array of claim24 further comprising a wafer of nonlinear optical material disposedbetween two mirrors positioned so as to form a Fabry-Perot resonatorwith a resonant frequency coincident with the mode of oscillation of themicrolasers and positioned so as to be irradiated by said microlasersand thereby stimulated into optical frequency conversion.
 26. Solidstate, optically pumped microchip laser comprising:a solid state gainmedium disposed between two mirrors, the distance between the mirrorsselected so that the gain bandwidth of the gain medium is less than orsubstantially equal to frequency separation of the cavity modes and suchthat one cavity mode frequency falls within the gain bandwidth of themedium; and an apparatus adapted for changing the temperature of saidgain medium and thereby thermally tuning said laser.
 27. Solid state,optically pumped microchip laser comprising:a solid state gain mediumdisposed between two mirrors, the distance between the mirrors selectedso that the gain bandwidth of the gain medium is less than orsubstantially equal to the frequency separation of the cavity modes andsuch that one cavity mode frequency falls within the gain bandwidth ofthe medium; and an apparatus adapted for applying a longitudinal stressto said gain medium and thereby stress tuning said laser.
 28. Solidstate, optically pumped microchip laser comprising:a solid state gainmedium disposed between two mirrors, the distance between the mirrorsselected so that the gain bandwidth of the gain medium is less than orsubstantially equal to the frequency separation of the cavity modes andsuch that one cavity mode frequency falls within the gain bandwidth ofthe medium; and an apparatus adapted for applying a transverse stress tosaid gain medium and thereby stress tuning said laser.